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Reperfusion injury
Reperfusion injury refers to damage to tissue caused when blood supply returns to the tissue after a period of ischemia. The absence of oxygen and nutrients from blood creates a condition in which the restoration of circulation results in inflammation and oxidative damage through the induction of oxidative stress rather than restoration of normal function. Mechanisms of reperfusion injury The damage of reperfusion injury is due in part to the inflammatory response of damaged tissues. White blood cells carried to the area by the newly returning blood release a host of inflammatory factors such as interleukins as well as free radicals in response to tissue damage .The restored blood flow reintroduces oxygen within cells that damages cellular proteins, DNA, and the plasma membrane. Damage to the cell's membrane may in turn cause the release of more free radicals. Such reactive species may also act indirectly in redox signaling to turn on apoptosis. Leukocytes may also build up in small capillaries, obstructing them and leading to more ischemia. Reperfusion injury plays a part in the brain's ischemic cascade, which is involved in stroke and brain trauma. Repeated bouts of ischemia and reperfusion injury also are thought to be a factor leading to the formation and failure to heal of chronic wounds such as pressure sores and diabetic foot ulcers . Continuous pressure limits blood supply and causes ischemia, and the inflammation occurs during reperfusion. As this process is repeated, it eventually damages tissue enough to cause a wound. In prolonged ischemia (60 minutes or more), hypoxanthine is formed as breakdown product of ATP metabolism. The enzyme xanthine dehydrogenase acts in reverse, that is as a xanthine oxidase as a result of the higher availability of oxygen. This oxidation results in molecular oxygen being converted into highly reactive superoxide and hydroxyl radicals. Xanthine oxidase also produces uric acid, which may act as both a prooxidant and as a scavenger of reactive species such as peroxinitrite. Excessive nitric oxide produced during reperfusion reacts with superoxide to produce the potent reactive species peroxynitrite. Such radicals and reactive oxygen species attack cell membrane lipids, proteins, and glycosaminoglycans, causing further damage. They may also initiate specific biological processes by redox signaling. Treatment Glisodin, a dietary supplement derived from superoxide dismutase (SOD) and wheat gliadin, has been studied for its ability to mitigate ischemia-reperfusion injury. A study of aortic cross-clamping, a common procedure in cardiac surgery, demonstrated a strong potential benefit with further research ongoing. Therapeutic Hypothermia An intriguing area of research demonstrates the ability of a reduction in body temperature to limit reperfusion injuries. This procedure is called therapeutic hypothermia. However, the therapeutic effect of hypothermia does not confine itself to metabolism and membrane stability. Another school of thought focuses on hypothermia’s ability to prevent the injuries that occur after circulation returns to the brain, or what is termed reperfusion injuries. In fact an individual suffering from an ischemic insult continues suffering injuries well after circulation is restored. In rats it has been shown that neurons often die a full 24 hours after blood flow returns. Some theorize that this delayed reaction derives from the various inflammatory immune responses that occur during reperfusion.Adler, Jerry. “Back From the Dead.” Newsweek. July 23, 2007. These inflammatory responses cause intracranial pressure, pressure which leads to cell injury and in some situations cell death. Hypothermia has been shown to help moderate intracranial pressure and therefore to minimize the harmful effect of a patient’s inflammatory immune responses during reperfusion. Beyond this, reperfusion also increases free radical production. Hypothermia too has been shown to minimize a patient’s production of deadly free radicals during reperfusion. Many now suspect it is because hypothermia reduces both intracranial pressure and free radical production that hypothermia improves patient outcome following a blockage of blood flow to the brain.Polderman, Kees H. “Application of therapeutic hypothermia in the ICU.” Intensive Car Med. (2004) 30:556-575. See also * Ischemia * Therapeutic hypothermia * Myocardial infarction -- Reperfusion References External links * Reperfusion Injury and "No Reflow" * Docs Change the Way They Think About Death Category:Stroke Category:Neurotrauma